1. FIELD OF THE INVENTION
The present invention relates to a method and a system for inspection of the inside of a nuclear reactor vessel, and more particularly to a method and a system for appropriately moving an inspection device within a reactor vessel to permit desired inspection of a part to be inspected in the reactor vessel.
2. DESCRIPTION OF THE PRIOR ART
Nuclear reactors have to be subjected to emergency shut-down when some accident happens in the reactors. The periodical inspection of the inside of the reactor vessel is required to avoid such an unexpected emergency shut-down, and to assure a safety operation of reactor. The inspection of the inside of the reactor vessel is made on such defects as a crack, reduction of thickness, deformation, impression and corrosion, which are generated at the inner wall of a reactor pressure vessel, piping, nozzle portion, or the like.
In nuclear reactors of the pressurized water type, all the structures contained within a pressure vessel can be removed from within the vessel, and therefore it is relatively easy to develop an automatic inspecting operation.
While, nuclear reactors of the boiling water type have a construction in which only a portion of structures contained in a pressure vessel can be taken out of the vessel. Specifically, the nozzle portion of the pressure vessel which is one of the most important inspection targets, is equipped with a feed water sparger and a pipe arrangement for reactor core spray, and the inner wall of the vessel is provided with a guide rod fastened thereto. That is, many obstacles have to be left within the pressure vessel. Therefore, in inspecting the inside of a reactor pressure vessel, it is required to employ a satisfactory monitoring system, and to look to the safety in operations.
At the nozzle portion of the reactor pressure vessel of the boiling water type, the coolant of a high temperature within the vessel comes in contact with the coolant of a low temperature from outside the vessel, and thus cracks are apt to be generated thereat as a result of fatigue due to repetitive thermal stress caused by temperature vibration. Accordingly, inspection of the nozzle portion is specifically required. However, the nozzle portion of boiling water reactor is different in shape from that of pressurized water reactor, and is provided with a corner section having the form of a saddle. For the inspection of such a corner section, it is required to move an inspection device along a complicated 3-dimensional curve, and to support a working unit for inspection by an arm having a large degree of freedom. Further, a control system for the inspection device is necessary to prevent a malfunction which permits damage of parts and elements through collision. Because of such problems, inspection of the pressure vessel of boiling water reactor has been made by persons who get into the vessel to effect inspecting operations. However, the working environment is extremely harmful within the pressure vessel because the inside of the vessel is irradiated with radioactive rays of high level, and is held at a high temperature and a high humidity. Therefore, early automation of inspecting operations is earnestly desired.
Remote control systems for inspecting the inside of nuclear reactor vessel are disclosed in U.S. Pat. No. 3,809,607 entitled "Reactor vessel in-service inspection assembly" and patented on May 7, 1974, in Japanese Patent Application Kokai (Laid-Open) No. 127094/75 entitled "Nuclear reactor vessel testing mechanism" and laid open on Oct. 6, 1975, in Japanese Patent Application Kokoku (Post-Examination Publication) No. 5872/77 entitled "Remote control device for defect detector for inner wall surface of cylindrical vessel" and published on Feb. 17, 1977, and in Japanese Utility Model Application Kokai (Laid-Open) No. 138099/78 entitled "Emergency disengaging device for manipulator arm of reactor vessel testing apparatus" and laid open on Nov. 1, 1978.
Although these patent and applications disclose a device for remotely inspecting the inside of a nuclear reactor vessel, they fail to disclose a monitoring device which monitors the operation of an inspection device to assure appropriate inspection.
The main item of inspection of the inside of reactor pressure vessel includes visual inspection of deformation, wrong mounting, or the like of structures within the vessel, and detection of defects at the inner wall of the pressure vessel and the nozzle portion. As important defect detecting methods, there are known the supersonic test for detecting inner defects and reduction of thickness and the fluid penetration test for detecting surface defects. Each test has its own feature. In the supersonic test, a medium is indispensable which propagates supersonic waves between the surface of an object under examination and a contact. The use of supersonic flaw detector in the water is advantageous, since the water serves as the supersonic wave propagating medium. Accordingly, a supersonic flaw detector to which the present invention is applied, has such a structure as permitting underwater operation.
The fluid penetration test employs a penetrating fluid, and therefore has to be carried out in the air. With a penetration defect detecting apparatus there are automatically conducted the application and wipe of three kinds of fluids including cleaning, penetrating and developing fluids, the brushing operation (which is a pretreatment) for scraping scales on the surface of an object under examination, and the visual examination of a film made of the developing fluid. When various units for conducting these operations are gathered up into a single unit, the penetration defect detecting apparatus becomes large in volume, and cannot be used in a narrow space of the nozzle portion. Therefore, the apparatus is divided into several kinds of operation or working units, which are mounted on a manipulator capable of getting near the object under examination and are exchanged in accordance with the contents of operation. As another defect detecting method assuming such a system, there is known the electric resistance test in which four electrodes are kept in contact with a surface under examination, an electric current is applied between two of these electrodes, and other two electrodes are used to measure a voltage distribution, in order to detect a change in voltage distribution caused by a defect existing between the measuring electrodes, if any.